DESCRIPTION: B23 is a 38 kDa protein found in the nucleolar preribosomal RNP particles where it is assumed to be a ribosome assembly factor. The proposed experiments focus on the structure-function relationships in a putative ribosome assembly factor called B23. Two isoforms of the protein B23.1 and B23.2 differ only in their C-terminal ends. B23.1 has nucleic acid binding activity, nuclear localization signal, and ribonuclease activity. B23.2 lacks the nucleic acid binding ability but retains the other two activities. The general regions and specific amino acid residues responsible for these three activities will be identified by deletion- and site directed mutagenesis. The mutated forms of the B23 protein will be expressed in bacteria. The mutant proteins will be purified using chromatographic methods. The activity of the mutant proteins will be measured: nuclear localization by equilibrium dialysis; nucleic acid binding by using nitrocelluose filter binding assays; ribonuclease activity will be measured by perchloric acid precipitation assays using in vitro transcribed RNA.